In order to improve communication quality and speed further to cope with an abrupt increase in mobile data traffic in recent years, standardization of a carrier aggregation (CA) function of allowing a radio base station (eNode B (eNB)) and a radio terminal (user equipment (UE)) to communicate with each other using a plurality of cells has been discussed in 3GPP LTE (Long Term Evolution). The cells that a UE can use in the CA are limited to cells of one eNB (i.e., cells served by one eNB).
The cells used by the UE are classified into a primary cell (PCell) which has already been used as a serving cell at the start of CA and a secondary cell (SCell) which is used additionally or subordinately. Non-Access Stratum (NAS) mobility information, security information (security input), and the like are sent and received through the PCell during radio connection (re)-establishment (RRC connection Establishment/Re-establishment) (see Non-Patent Literature 1). A DL carrier corresponding to the PCell is a DL primary component carrier (DL PCC) and its corresponding UL carrier is an UL PCC. Similarly, a DL carrier corresponding to the SCell is a DL secondary component carrier (DL SCC) and its corresponding UL carrier is an UL SCC.
When a UE that has been executing CA performs an inter-base station handover (Inter-eNB handover), the SCell(s) is released (see Non-Patent Literatures 2 and 3). In the following, with reference to a sequence diagram of FIG. 24, a procedure in which a UE that has been executing CA on Cell1 and Cell2 of a source eNB1 performs a handover to Cell3 of a target eNB2 will be described.
In step S1, a UE performs CA using Cell1 of eNB1 as a PCell and using Cell2 of the same eNB1 as a SCell. In step S2, the UE transmits a terminal measurement report (Measurement report) to the eNB1. In this example, the measurement report includes measurement results of Cell3 that serves as a handover target.
In step S3, the eNB1 sends a handover request (handover (HO) preparation request) to the eNB2. The HO preparation request includes a list (sourceSCellConfigList) of SCell(s) (in this example, the Cell2) which is used for CA in the eNB1. The HO preparation request may include a list (candidateCellInfoList) of candidate cell(s) used after the handover. In step S4, the eNB2 sends to the eNB1 an acknowledgement response (HO preparation request ACK) to the handover request. The HO preparation request ACK includes a list (sCellToReleaseList) of released SCell(s) (in this example, the Cell2). The HO preparation request ACK may include a list (sCellToAddModList) of SCell(s) (in this example, Cell4) to be used after the handover.
In step S5, the eNB1 transmits a message (RRC Connection Reconfiguration) instructing the UE to perform a handover to the Cell3 of the eNB2. At this time, the sCellToReleaseList, the sCellToAddModList, and the like, which has been sent from the eNB2 to the eNB1, are also transmitted to the UE. In step S6, the eNB1 sends a communication status of the UE to the eNB2 (SN status transfer). In step S7, the UE releases the SCell (i.e., the Cell2) (SCell release). In step S8, the UE synchronize with the eNB2 in the Cell3 and completes the handover process (Synchronization and RRC Connection Reconfiguration Complete). In step S9, when the UE has been instructed to add the Cell4 as SCell, the UE executes the request (SCell addition).
In step S10, the eNB2 sends a path switching request to a core network (EPC) (Path switch request). In step S11, the EPC executes path switching (Path switch). In step S12, the EPC notifies the eNB2 of completion of path switching (Path switch request ACK). In step S13, the eNB2 notifies the eNB1 of the release (acknowledgement of release) of terminal information (UE context release). In step S14, when the eNB2 has instructed the UE to add the Cell4 as the SCell, the eNB2 instructs the UE to activate the Cell4 (SCell activation). In step S15, the UE performs CA using the Cell3 as the PCell and using the Cell4 as the SCell.
Moreover, a concept of Inter-eNB CA of aggregating a plurality of cells served by different eNBs has been proposed (Non-Patent Literature 4). For example, the Inter-eNB CA may use a macro cell served by a macro base station (Macro eNB (MeNB)) and a pico cell served by a pico base station (Pico eNB (PeNB)).
Further, a method has been proposed in which signals for control-plane including mobility management of a UE are transmitted and received using a macro cell having a wide coverage and data-plane signals such as user data are transmitted and received using a pico cell which provides relatively better communication quality (Non-Patent Literature 5). This method is referred to as C/U Split.